Pulegone, para-menth-4(8)ene-3-one, is a compound which occurs naturally in plants of the family Labiatae and essential oils obtained therefrom. Included within this family of plants is the genus Mentha and the species piperita. Mentha piperita is commonly known as peppermint. It contains pulegone at levels of approximately one to five percent in the essential oil obtained therefrom. Pulegone itself is an oily liquid having a boiling point of 224 degrees Centigrade and a specific gravity of 0.94. The toxicity of pulegone has been investigated and it has been found to have an LD.sub.50 of 20 mg/kg of body weight per day. This is considered to be excessively toxic as indicated by the following references, so that legislation has been proposed to limit pulegone intake.
(1) F. Grundschober, "Perfume." Flavorist 4, 15 (1979); PA0 (2) Thorup, et al., Toxicology Letters 19, pg 207-210 (1983); PA0 (3) EEC, Council de l'Europe, International Organization of the Flavour Industry Committee of Experts. PA0 (2) Quantitative Determination of Minor Components in Essential Oils: Determination of Pulegone in Peppermint Oils, by Bicchi and Frattini, Journal of Chromatography 190, 471-474 (1980); PA0 (3) Short Term Toxicity Study in Rats Dosed with Pulegone and Menthol, by Thorup, et al., Toxicology Letters 19, 207-210 (1983); PA0 (4) Metabolism of Monoterpenes: Demonstration that (+)-cis-Isopulegone, not Piperitenone, is the Key Intermediate in the Conversion of (-)-Isopiperitenone to (+)-Pulegone in Peppermint (Mentha piperita), by Croteau and Venkatachalam, Archives of Biochemistry and Biophysics 249, No. 2, 306-315 (September 1986); and PA0 (5) Monoterpene Interconversions: Metabolism of Pulegone by a Cell-Free System From Mentha Piperita, by J. Battaile et al., Phytochemistry, 1968, Vol. 7, pp. 1159 to 1163, which reports a cell-free system study done primarily to identify the normal biosynthetic route of monoterpene interconversions in the peppermint plant itself.
It would accordingly be highly desirable to eliminate or at least substantially reduce the pulegone content of peppermint oil but, of course, any such elimination or reduction would have to be effected without detracting from the desirable and noble characteristics of the peppermint oil itself. This is easier said than done. However, the present invention provides just such a solution to the problem using a sterospecific reduction by hydrogenation process which is not only productive of the desired reduction in pulegone content as well as an increase in the desirable components menthone and menthol, but also an additional unpredictable advantage of enhanced stability against breakdown of menthofuran by oxidation or the like, menthofuran also being a natural ingredient of peppermint oil which, in the small amounts in which present, contributes to the natural properties and characteristics of the oil itself, as do numerous other components thereof which are also present in limited amounts.